Concerted Proton-Electron Transfer in the Oxidation of Hydrogen-Bonded Phenols
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- 作者：Ian J. Rhile ; Todd F. Markle ; Hirotaka Nagao ; Antonio G. DiPasquale ; Oanh P. Lam ; Mark A. Lockwood ; Katrina Rotter ; and James M. Mayer
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：May 10, 2006
- 年：2006
- 卷：128
- 期：18
- 页码：6075 - 6088
- 全文大小：278K
- 年卷期：v.128,no.18(May 10, 2006)
- ISSN：1520-5126
文摘
Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCNwith various one-electron oxidants. The bases are a primary amine (-CPh2NH2), an imidazole, and apyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is thephenoxyl radical in which the phenolic proton has transferred to the base, 
OAr-BH+, a proton-coupledelectron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols,predominately from the driving force for proton movement. One-electron oxidation of the phenols occursby a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotopeeffects, and 
G
/G
. The data rule out stepwise paths involving initial electron transfer to form thephenol radical cations [+HOAr-B] or initial proton transfer to give the zwitterions [-OAr-BH+]. The rateconstant for heterogeneous electron transfer from HOAr-NH2 to a platinum electrode has been derivedfrom electrochemical measurements. For oxidations of HOAr-NH2, the dependence of the solution rateconstants on driving force, on temperature, and on the nature of the oxidant, and the correspondencebetween the homogeneous and heterogeneous rate constants, are all consistent with the application ofadiabatic Marcus theory. The CPET reorganization energies, 
= 23-56 kcal mol-1, are large in comparisonwith those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic,based on minimum values of Hrp derived from the temperature dependence of the rate constants. Theseare among the first detailed analyses of CPET reactions where the proton and electron move to differentsites.
OAr-BH+, a proton-coupledelectron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols,predominately from the driving force for proton movement. One-electron oxidation of the phenols occursby a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotopeeffects, and G/G. The data rule out stepwise paths involving initial electron transfer to form thephenol radical cations [+HOAr-B] or initial proton transfer to give the zwitterions [-OAr-BH+]. The rateconstant for heterogeneous electron transfer from HOAr-NH2 to a platinum electrode has been derivedfrom electrochemical measurements. For oxidations of HOAr-NH2, the dependence of the solution rateconstants on driving force, on temperature, and on the nature of the oxidant, and the correspondencebetween the homogeneous and heterogeneous rate constants, are all consistent with the application ofadiabatic Marcus theory. The CPET reorganization energies, = 23-56 kcal mol-1, are large in comparisonwith those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic,based on minimum values of Hrp derived from the temperature dependence of the rate constants. Theseare among the first detailed analyses of CPET reactions where the proton and electron move to differentsites.